The following invention relates to a solution and a method of using that solution for treating printed circuit boards to prevent smear in the manufacture thereof.
During the manufacture of printed circuit boards made of a fiberglass epoxy having top and bottom conducting copper layers sandwiching a glass epoxy layer, and including conductive layers within the sandwich, holes are drilled which are then electroless plated to create circuit paths from one copper layer to the other. When these holes are drilled, a phenomenon known as "smear" may result. Smear is a dielectric material which is deposited on the inner surfaces of the holes made by the drill. The smeared inner walls of the drilled-out holes resist electroless copper deposition and the smeared material may also effectively cover inner copper conductive layers in multi-layer boards creating a void or causing poor adhesion such that no electrical contact is made between the inner walls of the hole and the inner conductive layers.
Smear is an industry-wide problem in the manufacture of printed circuit boards, and many approaches have been suggested as solutions. The most common approach, called "desmear," is to attempt to remove the smear after the holes have been drilled. Smear removal generally takes the form of immersing the boards in a strong sulfuric acid bath to roughen or remove the smear and then subsequently rinsing the boards. This approach is expensive and leads to other problems such as residues left by the sulfuric acid which also interfere with the electroless deposit of copper in the printed circuit board holes. Other approaches use chemicals other than sulfuric acid; nevertheless in these processes, it is necessary to treat the board after the holes have been drilled and then to rinse the treatment chemical away. The rinsing step, in particular, may be extremely time consuming and the equipment needed for post drilling treatment of the boards for any type of smear removal process is very expensive.
The only other known attempts to control smear prior to drilling involve spraying ultracold liquid air or liquid nitrogen on the drill bit prior to drilling, or the immersion of the boards in a water bath during drilling. In both cases the theory is that if the temperature of the drill bit can be kept low enough, smear will not form. Neither of the above approaches are viable because the specific heat of the epoxy is such that it reaches a melting point quickly, even though the drill bit is cold.